Temperature of Dark Matter

  Temperature of Dark Matter

By- Anmol Subba (Alumni)

Wait what? Does dark matter have temperature? Does it mean that they radiate heat energy like our sun or earth and thereby has temperature? 

The sad truth is no. Dark matter are very ghostly matter they neither interact with anyone nor they radiate any form of energy. Therefore it is very difficult to detect them. So far we have evidences for dark matter particles but have not detected them in our observations or experiments. Anyways back to the temperature of Dark Matter, if by temperature we do not mean the heat energy then what does temperature of dark matter actually mean?

You might be surprised to know this but by temperature the physicists mean the speed of the dark matter at which they move around. The physical meaning of temperature has nothing to do with the temperature of dark matter. Physicists have so many theories that they run of things to name and just start naming whatever they like, only to get confused with their actual meanings. 

If the dark matter travel at relativistic speed i.e. at the speed very close to the speed of light then we call the dark matter to be hot dark matter and if the speed is very less than the speed of light i.e non relativistic  we call it cold dark matter or CDM but if the speed is between relativistic and non relativistic then we call it, guess what? warm dark matter ! 

Moving on to the big question now ? Do we have all these three types of dark matter in our universe? Is there any dark matter that is present more than the other? Long story short, we don't know with exact certainty. If we don't know then why are we dark matter into categories? let me start telling you a story. Physicsts couldn't detect dark matter but they knew it was there due to a lot of evidence like rotational curve, bullet clusters etc. So they started simulating the early universe with N-body simulation (what is that? no important right now but n body simulation is basically a simulation in computer by putting codes to mimic all the forces and laws of the universe), they put different velocities for dark matter, to get the idea whether the universe has more cold or warm or hot dark matter.


N body simulation of dark matter

The results were very astonishing. They concluded that dark matter are very slow i.e cold dark matter. The whole theory of dark matter has been the study of cold dark matter or CDM. The theory studying cold dark matter which physicists believe is true is called lambda CDM. The nature is not very easy to understand. It turns out that there are some phenomenon which the lambda CDM model is not able to explain. Now the researchers has also started making theories with WDM i.e warm dark matter. But the question still remains what is the velocity of dark matter? is it hot or warm or something beyond our imagination?

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